The colors in an image vary depending upon the nature of the light illuminating the image. A white sheet of paper, for example, should look yellow under a yellow light bulb. In general, this is because different sources of illumination (e.g., sunlight on a clear day, overcast sunlight, fluorescent light, incandescent light) produce light with different “power spectra,” that is, with light of different colors having different intensities.
Human visual perception, however, does not slavishly follow variations in illumination. The human eyes and brain, knowing that a sheet of paper is white, automatically adjust the “white balance” of the perceived image of the paper. Due to this white balancing, the human consciously perceives the paper as white even under a wide variety of illumination types.
To make a camera image look “natural,” the colors in the image must be adjusted to match the human brain's white-balancing procedures. Colors in the image are adjusted by adjusting the gains of the three primary colors in the camera's light sensors. Adjusting white-balance semi-manually is relatively easy: A pure white card is placed before the camera, and the camera zooms in until the white card occupies the camera's full field of view. A button is pushed which tells the camera to adjust its color gain until the image is pure white.
User-friendly cameras, of course, perform white-balancing automatically. To summarize some very complicated procedures, the camera makes an assumption about the colors in an image. For example, the camera often assumes that the average intensities of the primary color channels (e.g., Red, Green, and Blue) are equal in the image. These automatic white-balancing procedures have difficulty, however, with images that violate the camera's assumptions, for example, images that have an intrinsic color cast or that have a large monochromatic area, such as a close-up of a human face or a landscape with a wide stretch of blue sky. When an image violates the camera's assumptions about the colors in the image, the camera's white-balancing operations are thrown off and give an undesirable “color twist” to the image.